A superabsorbent polymer composition in general refers to a water-swellable, water-insoluble material capable of absorbing at least about 10 times its weight, and up to about 25 times or more of its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. The present invention relates to superabsorbent polymer compositions which absorb water, aqueous liquids, and blood.
A superabsorbent polymer is a crosslinked partially neutralized polymer that is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent material. Superabsorbent polymer compositions may include post-treatment of the superabsorbent polymer such as surface crosslinking, surface treatment, and other treatment. Superabsorbent polymer particles are particles of superabsorbent polymers or superabsorbent polymer compositions. The acronym SAP may be used in place of superabsorbent polymer, superabsorbent polymer composition, and particles herein. A comprehensive survey of superabsorbent polymer compositions, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCH, New York, 1998.
Commercially available superabsorbent polymer compositions include crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution.
A primary use of superabsorbent polymer compositions is in absorbent composites that are used in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. For fit, comfort, and aesthetic reasons, and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber in these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more superabsorbent polymer composition content is being used in these sanitary articles.
Fluid distribution in an absorbent composite is generally dependent on the amount of free liquid available for distribution, the structure and materials of the absorbent composite, and a time factor. However, current absorbent composites useful in absorbent cores of absorbent articles generally may have inadequate, or less than desirable, fluid distribution properties. Poor fluid distribution decreases the full utility efficiency of absorbent composites as not all of the superabsorbent polymer composition is absorbing liquid.
One potential solution known in the art for improving fluid distribution in an absorbent composite is to use superabsorbent polymer compositions having a slow absorbency rate. The theory for using slow absorbency superabsorbent polymer compositions is that there would be diminished or delayed gel blocking and thereby would provide more free liquid and time for the liquid to distribute away from the insult target area. However, although distribution may be enhanced using the slow superabsorbent, the absorbent composite typically does not provide necessary leakage protection.
As may be seen in the use of slow absorbency superabsorbent polymer compositions, there is a conflict between the functions of lockup and distribution. One problem is that the time required to lockup liquid into the superabsorbent polymer composition and open up composite structure by SAP swelling is relatively long. It is this free liquid which is in the target area during fluid insult period that is difficult to get into the absorbent product core and believed to be responsible for leakage of liquid from an absorbent product while in use. To reduce leakage, the superabsorbent polymer composition needs to lockup liquid at a rate similar to the liquid delivery rate so that an absorbent product has adequate fluid handling functionality. However, if the superabsorbent polymer compositions absorb the liquid, then distribution of liquid is reduced since there is no free liquid to be distributed after fluid insult. Using superabsorbent polymer compositions that have the ability to quickly lockup liquid and then gradually release this liquid so that it may be distributed will result in desirable leakage and distribution behavior.
There is a need for a superabsorbent polymer composition that allows enhanced fluid distribution properties while maintaining other absorbent properties, thereby maximizing the absorbent capabilities of superabsorbent polymer composition.